The aldol condensation reaction of saturated aldehydes with elimination of water to give the corresponding unsaturated dimer or α,β-unsaturated aldehyde is a reaction which is familiar in organic chemistry. The conversion can be effected with only one particular aldehyde compound in a self-condensation reaction, or else between aldehydes with a different number of carbon atoms in a mixed aldol condensation reaction. In order that the addition of two aldehyde molecules to give the β-hydroxyaldehyde can be followed by the intramolecular elimination of water and hence the formation of the α,β-unsaturated bond, at least one α-aldehyde bearing two hydrogen atoms on the α-carbon atom to the carbonyl group must be present in the aldol condensation reaction.
The deliberate suppression of the elimination of water from the β-hydroxyaldehyde intermediate is, if anything, restricted to special cases. According to WO 95/07254 A1, the aldol addition of n-butyraldehyde is performed with aqueous sodium hydroxide solution in the presence of polyethylene glycol at low temperatures. After neutralization of the basic catalyst and removal of the aqueous phase, the organic phase comprising 2-ethyl-3-hydroxyhexanal is hydrogenated. The target product desired is 2-ethyl-1,3-hexanediol, and the formation of 2-ethylhexanol is to be suppressed as far as possible.
Of greater industrial significance, however, is the preparation of 2-ethylhexanol by the aldol condensation reaction. The alcohol is prepared proceeding from n-butyraldehyde in the presence of an alkaline catalyst, typically an aqueous sodium hydroxide solution, at elevated temperature via the 2-ethylhexenal intermediate formed with elimination of water. The α,β-unsaturated aldehyde is subsequently hydrogenated to give 2-ethylhexanol, typically in the gas phase in a first stage with a subsequent liquid phase hydrogenation in the second stage (EP 0 420 035 A1). The water of reaction released dilutes the aqueous sodium hydroxide solution, which is likewise contaminated with organic impurities such as butyrates. Therefore, a portion of the diluted and contaminated aqueous sodium hydroxide solution constantly has to be discharged and replaced by fresh alkali. The aldol condensation of n-butyraldehyde in the presence of an aqueous sodium hydroxide solution can be performed, for example, in a stirred tank as described in U.S. Pat. No. 3,763,247 or DE 927 626, or in a packed column operated in countercurrent [G. Dümbgen, D. Neubauer, Chemie-Ing.-Tech. 41, 974 (1969)]. However, reaction in a stirred tank is disadvantageous since, in this type of reaction design, heat removal can be problematic and the operation of mechanically moving parts requires intensive maintenance and frequent repairs.
It is also known that n-butyraldehyde and aqueous sodium hydroxide solution can be mixed vigorously in a mixing pump, and the heterogeneous mixture can be left in the turbulent state after leaving the mixing pump. According to U.S. Pat. No. 2,468,710, the mixture is conducted through a mixing zone and then passed, for completion of the reaction, into a reaction vessel from which the desired aldol condensation product is removed at the top, and from which a liquid stream is recycled via a bottom draw. According to GB 761,203, the heterogeneous mixture is conducted through a flow tube in a turbulent manner by means of the mixing pump, and then introduced into a phase separator. The lighter organic phase is removed, while the aqueous sodium hydroxide solution is partly discharged and partly recycled back into the aldol condensation process. The known process can also be applied to the mixed aldol condensation reaction of different aldehydes, for example to the reaction of acetaldehyde with n-butyraldehyde, which leads to a mixture of α,β-unsaturated aldehydes in which the compounds from the self-aldolization and mixed aldolization are present.
EP 1 106 596 A2 concerns the performance of the aldol condensation reaction in a tubular reactor. The known process is characterized by the specification of a load factor which is the minimum that should be established to ensure the turbulent operation of the tubular reactor. A characterizing feature is the high ratio of the mass flow rate of the aqueous catalyst phase to that of the aldehyde phase, such that the aqueous catalyst phase forms the continuous phase, in which the aldehyde phase is present dispersed in fine droplets. The aqueous catalyst phase may optionally also comprise a water-soluble solvent, such as diethylene glycol, in order to facilitate mass transfer between the aqueous catalyst phase and the organic aldehyde phase. By way of example, the aldol self-condensation of n-pentanal or 3-methylbutanal is discussed, as is the aldol condensation of an aldehyde mixture composed of n-pentanal and 2-methylbutanal.
The aldol condensation of aliphatic pentanals to give α,β-unsaturated C10-aldehydes is likewise described in WO 2010/105892 A1. In this process too, a tubular reactor is employed, the organic phase being dispersed in the aqueous catalyst phase in the form of droplets. The average Sauter diameter of the droplets is between 0.2 and 2 mm. This droplet size can be established with a low energy input and at the same time ensures high mass transfer between the dispersed organic phase and the continuous aqueous catalyst phase.
The known processes for performing the aldol condensation to give α,β-unsaturated aldehydes require a high energy input to operate a flow tube or a tubular reactor in the turbulent state. The prior art either proposes the use of mixing pumps with a downstream flow tube or recommends, in the case of a tubular reactor, a high ratio of the mass flow rates of the aqueous catalyst solution to the aldehyde phase supplied. A disadvantage in this process is, however, the use of a very high mass flow rate of aqueous catalyst solution, such that only a small amount of organic product is conducted through the reactor per unit time and volume. If a water-soluble solvent is used in addition, the purification complexity increases for the desired product. In the case of use of interface-active substances too, increased contamination of the aldolization wastewater discharged with organic impurities is to be expected. The operation of mixing pumps is energy-intensive and increases the maintenance expenditure. The aldol condensation reaction of higher aldehydes, such as C5-aldehydes and higher, likewise requires a comparatively high reaction temperature in order to achieve very substantial elimination of water from the β-hydroxyaldehyde intermediate to give the α,β-unsaturated aldehyde. If the desired target product is to result from the mixed aldolization of two different aldehydes, the reaction conditions should be adjusted such that the self-condensation of the respective starting aldehydes is reduced to a very minor level.
It is therefore an object of the present invention to provide a process for performing polyphasic aldol condensation reactions to give mixed α,β-unsaturated aldehydes, which requires a low level of technical complexity and is of low energy intensity, and which features a high space-time yield of mixed aldol condensation products or α,α-unsaturated aldehydes. In addition, the risk potential attendant to working with lower aldehydes which are of marked volatility and have a low ignition temperature is to be minimized. The mixed aldol condensation products are likewise to be obtained with high conversion and high selectivity, and the formation of α,β-unsaturated aldehydes from the self-condensation and formation of by-products, more particularly the formation of high-boiling condensation products, is to be suppressed.